RAID

Question 1

What does RAID stand for?

Answer 1

Redundant Array of Independent Disks, used to stand for Redundant Array of Inexpensive Disks but industry changed it.

Question 2

What is the ‘enemy’ of RAID

Answer 2

SLED - Single Large Expensive Disk

Question 3

Define Striping

Answer 3

Distributing data over multiple drives

Question 4

RAID Level 0

Answer 4

Simply stripes data across drives.
Works best with large requests
Not true RAID because there is no redundancy.
Another disadvantage of this organization is that the reliability is potentially worse than having a SLED.

Question 5

RAID Level 1

Answer 5

True RAID. It duplicates all the disks.
On a write, every strip is written twice.
On a read, either copy can be used, distributing the load over more drives. Consequently, write performance is no better than for a single drive, but read performance can be up to twice as good.
Fault tolerance is excellent: if a drive crashes, the copy is simply used instead. Recovery consists of simply installing a new drive and copying the entire backup drive to it.

Question 6

RAID Level 2

Answer 6

RAID level 2 works on a word basis, possibly even a byte basis.
Imagine splitting each byte of the single
virtual disk into a pair of 4-bit nibbles, then adding a Hamming code to each
one to form a 7-bit word, of which bits 1, 2, and 4 were parity bits.
Further imagine
that the seven drives were synchronized in terms of arm position
and rotational position. Then it would be possible to write the 7-bit Hamming
coded word over the seven drives, one bit per drive.

Question 7

RAID Level 3

Answer 7

RAID level 3 is a simplified version of RAID level 2.
Here a single parity bit is computed for each data word and written to a parity drive.
It provides full 1-bit error correction since the position of the bad bit is known.

Question 8

RAID Level 4

Answer 8

RAID level 4 works with strips again, not individual words with parity, and do not require synchronized drives.
RAID level 4 is like RAID level 0, with a strip-for-strip parity written onto an extra drive.
For example, if each strip is k bytes long, all the strips are XOR’ed together, resulting in a parity strip k bytes long. If a drive crashes, the lost bytes can be recomputed from the parity drive.

Question 9

RAID Level 5

Answer 9

RAID level 5 works with strips again, not individual words with parity, and do not require synchronized drives.
RAID level 5 by distributes the parity bits uniformly over all the drives, round robin fashion.